1. Field of the Invention
The present invention relates to an optical recording medium and an optical recording method. The present invention allows the optical recording medium to be irradiated with a laser beam at an irradiation time and/or an irradiation power, which is changed in multi-stages in response to data made available for recording, to form a recording mark corresponding to the irradiation power and/or irradiation time and thereby perform multi-level recording of the data.
2. Description of the Prior Art
There are a number of methods for recording data on a prior-art optical recording medium by changing the length of a recording mark (the length of a reflected signal modulating portion) in multi-stages. In contrast, a number of studies have also been made on a method for recording a plurality of data on regions of the same length by changing the depth of the recording mark (the degree of modulation of a reflected signal) in multi-stages.
According to this optical recording method, a plurality of data can be recorded in the direction of depth in comparison with the recording of binary data using simply the presence and absence of pits. Thus, this makes it possible to increase the amount of signals to be assigned to a given length, thereby allowing the linear recording density to be improved. Accordingly, such optical recording methods have been suggested which make use of hologram or employ multi layers as the recording layer.
It is herein referred to as multi-level recording to record data in multi-stages using such as variations in depth of reflectivity.
It is necessary to make the recording mark short in order to improve the recording density in such multi-level recording.
However, it is made difficult to perform multi-level recording with the recording mark being made less in diameter than a focused laser beam to be used for recording and reading.
For example, described in Japanese Patent Laid-Open Publication No. Hei 10-134353 is that the amount of laser beam is adjusted to perform multi-level recording. In this publication, with dye film or phase changing film being employed as the recording medium, read signals are formed based on a difference in reflection between recorded and non-recorded portions. Therefore, the method according to Japanese Patent Laid-Open Publication No. Hei 10-134353 relates the non-recorded stage and recorded stage to the absence and presence of records, thus being not suitable for multi-level recording. More specifically, the phase changing film and the dye film provide no intermediate state between the recorded and non-recorded states.
On the other hand, for example, disclosed in Japanese Patent Laid-Open Publication No. Hei 1-182846 is an optical recording medium that allows the optical absorbance of reactants in the recording layer to vary digitally when an amount of light is incident digitally on the recording layer.
However, this optical recording medium is expected to have an extremely small absolute value of variations in optical absorbance for the amount (the number of times) of irradiation of laser beam and thus has not yet been put to actual use.
Furthermore, disclosed in Japanese Patent Laid-Open Publication No. Sho 61-211835 is an optical recording method for changing the intensity or the number of times of irradiation of beam, with which a photochromic material is irradiated, to perform recording in given different stages of coloring density states.
However, there is a problem that this optical recording method cannot read the coloring density states in five stages or more upon irradiation with laser beam for reading.
On the other hand, a double density CD (DDCD) has been suggested which is a CD with an increased storage capacity. The DDCD is 12 cm in diameter, which is the same as the CD, and formatted to provide one side thereof with a capacity of 1.3 GB and thus a recording density twice as high as that of the CD.
In addition, the DDCD makes as-is use of a laser beam of wavelength 780 nm that is used by the current CD, condensing the beam with a high NA lens to reduce the recording mark in size.
According to the DDCD specifications, the track has a pitch as narrow as 1.1 xcexcm in comparison with 1.6 xcexcm of the current CD and is thus provided with a higher density. In addition, as a matter of course, the recording mark is reduced in size, as the shortest recording mark was conventionally 0.833 xcexcm in length but now is 0.623 xcexcm while the longest recording mark is about 2 xcexcm.
Accordingly, the DDCD can be readily implemented by making partial additions or modifications to the parts or firmware of the current CD. This allows the DDCD to feature almost no increase in fabrication costs. The DDCD is expected to find its use with personal computers as their external storage devices or the like.
However, suppose that each aforementioned type of storage medium and recording method is applied to the DDCD. As mentioned above, this would provide insufficient multi-level recording and require substantial changes to the parts and firmware of the current CD, presenting a problem of an increase in fabrication costs.
Furthermore, suppose that the binary recording method or a conventional recording method is performed on the recording medium narrower in track pitch than the conventional one. This would raise a problem that the same recording method as that of the current CD provides a so-called teardrop shaped recording mark due to heat accumulated upon recording. Thus, the recording mark having the aforementioned longest length or a length close to that is to have a teardrop shape with a tail end greater in width than the head end even when the recording laser beam spot is reduced in diameter to all manner of sizes.
As described above, the DDCD has a narrowed track pitch. This causes the teardrop shaped recording mark to produce a cross-talk signal from neighboring tracks and a cross-write phenomenon by which recording marks written on neighboring tracks are deformed. With the phase changing recording layer, this also causes a cross-erase phenomenon to increase and a so-called inter-symbol interference to increase by which the waveform of a read signal is subjected to an interference and distorted between the recording marks on neighboring tracks. Thus, the DDCD presents these new problems.
The inventor has found that multi-level recording can be performed in five stages or more by changing the time for irradiation with a laser beam even under the condition that the length of a recording mark is less than the diameter of a focused beam. The inventor filed a patent application of the invention (which has not yet been laid opened at the filing date of the application of the present invention).
The present invention made it possible to perform the aforementioned multi-level recording with the phase changing material and the dye material being employed as the material for the recording film.
In addition, the inventor made an attempt to perform multi-level recording in five stages or more by changing the time and/or power of irradiation of the laser beam.
It is therefore an object of the present invention to provide an optical recording medium and an optical recording method that make use of an optical recording medium such as a CD-R (CDxe2x80x94Recordable) and CD-RW (CDxe2x80x94Rewritable), which are generally in wide actual use. The medium and method are adapted to perform multi-level recording in multi-stages without substantially modifying the parts-and the firmware of the recorder, thereby making it possible to provide signals of good quality.
It is another object of the present invention to provide an optical recording medium and an optical recording method which prevent the recording mark from taking a teardrop shape in the DDCD-R (DDCDxe2x80x94Recordable) and DDCD-RW (DDCDxe2x80x94Rewritable). It is thereby made possible to perform recording at a predetermined recording density without increasing cross-talk signals, cross-write, cross-erase, and inter-symbol interference.
The inventor has made intensive studies on optical recording media and found a recording method for performing multi-stage recording on an organic dye recording layer. Thus, the inventor has confirmed that the recording method makes it possible to perform five stages or more of multi-level recording on an optical recording medium having an organic dye recording layer. The inventor has also found a recording method for modulating in multi-stages the optical reflectivity of an entire virtual recording region with a given area including a region having a high optical reflectivity around a recording mark. The inventor has confirmed that the recording method makes it possible to perform five stages or more of high-density multi-level recording on an optical recording medium. In particular, it has been confirmed that a good multi-level recording can be performed by setting virtual recording cells (region) contiguously in a direction of relative displacement between the laser beam and the recording layer. The virtual recording cells are provided with an arbitrary unit length in the displacement direction and a unit width in the direction orthogonal thereto, and set along a track having a track pitch defined to be 1 to 1.5 xcexcm in the orthogonal direction.
In other words, the present invention described below has made it possible to achieve the aforementioned objects.
(1) An optical recording medium for recording information by irradiating a recording layer with a laser beam to form a recording mark thereon and for reading recorded information by irradiating the recording mark with a reading laser beam, said optical recording medium comprising a virtual recording cell set contiguously on said recording layer along a track in a direction of relative displacement between the laser beam and said recording layer, the virtual recording cell being provided with a given unit length in the direction and a unit width in a direction orthogonal thereto, the track being provided with a track pitch defined to be 1 xcexcm to 1.5 xcexcm in the orthogonal direction, wherein a recording mark can be formed on said recording layer in the virtual recording cell, said recording mark having at least different one of a size and an optical transmittance corresponding to five stages or more of modulation of an irradiation time and/or irradiation power of the laser beam, and optical reflectivity of the entire virtual recording cell is modulated to thereby enable multi-level recording of information in five stages or more in accordance with at least one of an area ratio of the recording mark to the virtual recording cell and an optical transmittance of the recording mark.
(2) The optical recording medium according to (1), wherein the unit length of said virtual recording cell is set to be generally equal to the length of a recording mark formed by a laser beam irradiation at a maximum time and/or a maximum irradiation power.
(3) The optical recording medium according to (1) and (2), wherein a groove for guiding a laser beam is provided along said recording layer, said virtual recording cell is set in said groove, and said unit width is set to be generally equal to the width of said groove.
(4) The optical recording medium according to any one of (1) to (3), wherein said unit length of said virtual recording cell is made equal to or less than a beam waist of said reading laser beam.
(5) The optical recording medium according to any one of (1) to (4), wherein multi-level recording of information is performed in advance on a portion of said recording layer.
(6) The optical recording medium according to any one of (1) to (5), wherein particular information indicative of being a multi-level recording medium is recorded on at least one of said virtual recording cell, a portion having the multi-level recording performed thereon and a wobble.
(7) The optical recording medium according to any one of (1) to (6), wherein a groove for guiding a laser beam is provided along said recording layer, and the groove is partly discontinued.
(8) The optical recording medium according to any one of (1) to (7), wherein said recording layer is formed of an organic dye.
(9) The optical recording medium according to any one of (1) to (8), wherein the unit width of said virtual recording cell is so set as to allow recording to be performed on the recording layer when the recording layer is irradiated with a laser beam of wavelength 780 nm via an objective lens having a numerical aperture of 0.55 or more with said spot diameter being 1.2 xcexcm or less.
(10) The optical recording medium according to any one of (1) to (9), wherein said virtual recording cell is arranged in a spiral fashion with a track pitch being from 1 xcexcm to 1.5 xcexcm and formed in a disc shape as a whole.
(11) An optical recording method for recording information by irradiating a recording layer with a laser beam, while one of the recording layer and the laser beam is displaced in a given direction relative to the other, to form a recording mark, comprising the steps of: defining a virtual recording cell being set contiguously on said recording layer in the direction of relative displacement, the virtual recording cell being set to be generally equal to the width of a groove formed at a track pitch of 1 xcexcm to 1.5 xcexcm in a direction orthogonal to the direction of displacement, modulating the irradiation power of the laser beam in five stages or more for each virtual recording cell, and performing multi-level recording of information by changing a size of the recording mark formed within the virtual recording cell to modulate the optical reflectivity of said entire virtual recording cell in accordance with the irradiation time and/or the irradiation power of said laser beam, said optical reflectivity being provided by at least one of an area ratio of the recording mark to the virtual recording cell and an optical transmittance of the recording mark.
(12) The optical recording method according to (11), wherein said recording layer is formed of a material for allowing at least one of the size and optical transmittance of the recording mark to be modulated only in accordance with the irradiation time provided when said beam diameter of the laser beam is made generally constant, and said recording layer is irradiated with the laser beam having a constant beam diameter.
(13) The optical recording method according to (11), wherein said recording layer is formed of a material for allowing at least one of the size and optical transmittance of the recording mark to be modulated only in accordance with the irradiation power provided when said beam diameter of the laser beam is made generally constant, and said recording layer is irradiated with the laser beam having a constant beam diameter.
(14) The optical recording method according to (11), (12) or (13), wherein said laser beam is a beam of light provided with a wavelength of 780 nm, and said recording layer is irradiated with said laser beam via an objective lens having a numerical aperture of 0.55 or more.
For example, in this invention, for the recording mark being made less than the recording beam diameter, the laser irradiating time and the recording power is modulated with the other thereof being kept at constant. At least one of the size of the recording mark and the optical transmittance is thereby modulated to vary in multi-stages the level of the optical reflectivity provided by the area percentage of the recording mark within a given region, thereby making it possible to perform multi-level recording.
Furthermore, this effect was particularly distinct upon performing five stages or more of multi-level recording.
In addition, for this multi-level recording, the multi-stage recording is discriminated from a difference in the level of the optical reflectivity provided by the area percentage of the recording mark within a given region. This makes it possible to increase the recording density and perform recording of the same recording capacity without using long recording mark, thereby reducing the f-longest recording mark in length.